PROJECT SUMMARY Systemic lupus erythematosus (SLE) is a complex autoimmune disease characterized by the presence of circulating autoantibodies to nucleic acids and to proteins with which they associate. Signaling through the nucleic acid sensing TLRs, TLR7 and TLR9, is critical in SLE pathogenesis, and dysregulated TLR signaling can promote lupus in humans and in mouse models. Plasmacytoid dendritic cells (pDC) and B cells both express these nucleic acid sensing TLR and are important in SLE pathogenesis. Autoreactive B cells produce pathogenic autoantibodies in SLE, and B cell antibody production is promoted by TLR7 and TLR9 signaling. pDC use TLR7 and TLR9 to respond to nucleic acids in immune complexes resulting in the secretion of large quantities of type I IFN cytokines, which have pleiotropic effects on the immune response, including enhancing dendritic cell (DC) maturation, plasma cell formation, and T cell responses, all of which can promote a feed forward loop of immune activation. Therefore, understanding the mechanisms by which TLR7 and TLR9 signaling are regulated in these two critical cell types is important for understanding the pathogenesis of SLE and in defining therapeutic targets for this disease. We have identified the signaling adapter B cell adapter for PI3-kinase (BCAP) as a key modulator of TLR signaling in multiple immune lineages. First, we found that in macrophages BCAP inhibits TLR-induced inflammatory cytokine production via activation of PI3-kinase. We recently showed that BCAP promotes pDC IFN?, but not IL-6, secretion. We have also begun to examine how BCAP regulates B cell TLR7/9 responses, an understudied area. Our preliminary data show that BCAP is a key regulator of B cell TLR7/9 responses in all B cell subsets, with a particularly striking decrease in proliferation and IgG secretion from splenic marginal zone B cells. Additionally, we have found that BCAP- deficiency protects the TLR7.1 mouse lupus model from disease. Together, our findings show an important role of BCAP in endosomal TLR signaling in pDC and B cells, both important in SLE pathogenesis. Given the importance of TLR7 and TLR9 signaling in both B cells and pDC in SLE, the premise of this application is that BCAP regulation of pDC and B cell TLR7/9 signaling is critical in the development of lupus-like disease. Specifically, we will 1) determine the mechanism by which BCAP regulates TLR7/9-induced IFN? production in pDCs, 2) determine the mechanism by which BCAP regulates B cell TLR7/9 responses, and 3) determine the relative contribution of BCAP in pDCs and B cells to lupus-like disease using two mouse models.